Pioneering study reveals how babies work out who they can trust

If you are willing to share an ice cream cone with another person, it is perhaps the ultimate marker of how comfortable you are with one another. Those who lick the same ball of frozen cream and sugar together, stay together, or something. But what makes one person an acceptable co-eater and another just too much of a stranger? And what does that kind of strong, close relationship behavior signal to others? That’s the question underpinning a new study — specifically, how swapping spit teaches children who to trust the most.

What’s new — Published on Thursday in the journal Science, researchers show how infants, toddlers, and children infer the “thickness” or intimacy of adult relationships around them using saliva sharing. Doing this helps these budding humans work out their place in the world, and particularly, within a family or community structure.

One of the questions this study set out to answer, Ashley Thomas, the study’s first author and a psychologist at the Massachusetts Institute of Technology, says was: “Do you know [how close two people are] because of years and years of experience with this, or is it something that's really easy for humans to pick up on in the environment?”

“What we found is that it seems to be very easily and quickly learned by infants,” she says.

It seems that kids expect that two people who kiss or share food are more likely to have a caregiver-caretaker relationship than two people who, say, play checkers.

Why it matters — Swapping spit may seem like an obvious sign of intimacy — so obvious that nobody’s thought to study it before. But beyond that, understanding how infants and young children work out their place in the world in terms of relating themselves to others is still an emerging field of study. Spit may be an instrumental way babies start to navigate the world and figure out who’s closest to them.

“What this work shows is that infants from very early on are figuring out not only who is connected, but how they’re connected,” says Thomas.

“That’s the thing we didn’t know before — whether infants were mapping their social world in this specific way.”

“An infant is just born into whatever environment they’re born into, they don’t get to choose, but they still have to figure it out,” Thomas says. “These types of cues might help them figure this out.”

Christine Fawcett, a psychologist at Uppsala University in Sweden, believes this ability may be an “inborn evolutionary mechanism” that helps babies survive by knowing who they can depend on for care. Fawcett was not involved in the study, but she wrote an accompanying article also published Thursday in Science.

Digging into the details — Thomas and her team studied how infants, toddlers, and young kids respond to different scenarios exemplifying two different relationships. In the experiment, children from these three different age groups watched two videos of a woman talking to a puppet. In one video, the woman ate a piece of orange and then passed that piece to share with the puppet. In the other, the woman passed a ball to the puppet.

In both videos, the puppet starts crying after receiving the orange or the ball. Thomas and her team tracked the childrens’ gaze to see how much attention they paid each woman and for how long as a measure of whether they expected the woman to help the puppet, or not. Most kids looked to the food-sharer as the woman they thought most likely to comfort the crying puppet.

A big caveat of this study is that it looks specifically at how babies perceive close relationships between strangers — and it doesn’t account for cultural differences. The study can’t definitively tell us how tots apply the same logic to inferring their own inner circle, or whether saliva-sharing is as important in working out social bonds for children raised in other cultural contexts outside the United States.

“There is still some variability and how sharing-y people are,” Fawcett says. “Some people are very kissy-kiss-kissy with their babies.”

Saliva isn’t the only indicator of social closeness, she points out. Regardless of how common sharing saliva is in a certain cultural context, it doesn’t mean the child will grow up with any social deficit, just that they will use another behavioral cue to infer thick relationships.

Saliva-sharing also bears no importance on how good a caretaker someone is, Thomas says. It’s merely an indicator of closeness.

“We don't expect daycare teachers to have the enduring attachments that you might find in a thick relationship,” she says. “That doesn't mean that they’re not doing a good job taking care of the children. Daycare teachers provide an absolutely essential service to society, and we should pay them more.”

What’s next — Thomas and her team want to see how if the hypothesis developed in this study holds up or changes in other cultural contexts. Specifically, she’s interested in how babies in different cultures might view the saliva stimuli or if their intuition changes during development according to cultural norms.

She also wants to investigate saliva-sharing in family contexts. “What changes when infants are watching people that they know, their parents or their siblings, or their grandparents or their daycare teachers, or nannies?” she says.

Fawcett, meanwhile, is interested in tracing the physiological responses underlying a child’s reaction, like pupil dilation, which can betray surprise. If two people have just met and then kiss on the mouth or share food, that might surprise an infant, for example.

So don’t be offended if a little kid doesn’t go to you for help or attention. Maybe first you should offer someone else some of your ice cream — it may help build more than trust.

Abstract: Across human societies, people form ‘thick’ relationships, characterized by strong attachments, obligations and mutual responsiveness. How do children identify thick relationships? People in thick relationships engage in distinctive interactions, like sharing food utensils or kissing, that involve sharing saliva. Here we show that children (N=113), toddlers (N=190), and infants (N=81) inferred that dyads who shared saliva (compared to other positive social interactions) had a distinct relationship. Children expected saliva sharing to happen in nuclear families. Toddlers and infants expected that people who shared saliva would respond to one another in distress. Parents (N=129) confirmed that saliva sharing is a valid cue of relationship thickness in their children’s social environments. The ability to use distinctive interactions to infer categories of relationships thus emerges early in life, without explicit teaching, allowing young humans to rapidly identify close relationships, both within and beyond families.